Scallop myosin is a regulated molecule that is switched on by the direct binding of Ca2+. The globular head portion of myosin (subfragment-1, S1) is the motor that contains the ATPase site and interacts with actin. The regulatory and the essential light chains, which form part of the head, act as inhibitory subunits. Scallop myosin is therefore uniquely suited for studies of the structural states that correspond to rest and activity, for characterizing the architecture of its unusual Ca2+-binding site, and for following light chain rearrangements and other conformational changes that result from Ca2+ binding. We have grown X-ray quality crystals of the 'native' regulatory domain of the scallop myosin head, consisting of a 10 kD fragment of the heavy chain and both light chains, and have obtained an interpretable electron density map of the structure. The atomic-level structure of the native regulatory domain will be determined by X-ray crystallography in the presence and absence of bound Ca2+ ions in order to describe the structure of the Ca2+-binding site, and to identify the sequences responsible for binding the light chains to the heavy chain. The regulatory domain has also been reconstituted from isolated chains and complete function is restored. We will attempt to crystallize and determine the structure of such domains reconstituted with mutant light chains and with light chains derived from both regulated and unregulated myosins. Such studies should clarify the mechanisms of light chain function. Small crystals of S1 have also been obtained, and we will attempt to improve these crystals in order to determine the structure at the atomic level. This result would allow us to describe the Ca2+- and ATP-induced structural changes of the myosin head. Preparation of a regulated S1 will also be attempted. The results of these studies will be applicable to the activation of all regulated myosins (smooth muscle and non-muscle myosins) and will help to clarify the structural changes involved in contraction.